So... it looks like they nerfed the free version and took out z80. Sorry about that...This isn't exactly breaking news, but I thought people should know about this. IDA v5.0 is free now and is by far the best way to disassemble OS's and files, the reason it's so good is simply because you make changes along the way.
Here is the download page for IDA.
A good way to start with IDA would be to disassemble your boot code. To get your boot code, take the .8xv that Rom8x gives you and cut off the 8xv data up front. The last two bytes you will remove will be $00, $40 and the first one you will save will be $3E. You'll also want to take off the two byte checksum at the end.
Once you have that, go into ida, X out the window that pops up and open your .bin file. From there, select processor = Zilog z80 and set 0000-7fff as rom and 8000-ffff as ram. Then set the loading address to $4000 and have fun.
After you've gotten used to the feel of IDA and now understand the purpose of the boot code better, you can try to disassemble the OS. The problem with OS's is that they need to be broken up into several pages. You can do this manually, or you can use
BrandonW's Pterodactyl which will do it for you. After you have the pages, just load each on individually and start disassembling. Also, with Pterodactyl, it will make a .idc file for each page. If you run that on the correct page, it will mark ram with the labels in ti83plus.inc as well as marking OS routines in the code.
Well, have fun. Hopefully this will lead to a greater knowledge of the operating system and perhaps a few more cool projects in the future. (It might also show you why Ti sucks at coding
)